Find and document this prefetching magic that happens when rendering an entity page (either for the whole entity or just terms / labels)

We all know that this prefetching of labels for rendering a wiki page happens somewhere, we should document where and also how federated properties requires / relies on it to avoid multiple API calls.

This can be documented in the section for prefetching-term-lookup-callback in extensions/Wikibase/docs/topics/entitytypes.md. At a later point we might want to add documentation specific to federated properties.